Neurons are specialised excitable cells that receive process and transmit information through electrical and chemical signals. They are fundamental functional units of the Nervous System, although nervous system function also depends heavily on glial cells and interactions with other tissues. A typical neuron contains a cell body or soma, dendrites that commonly receive inputs, and an axon that can conduct electrical signals towards other cells. Neurons vary enormously in shape size connectivity and function.
Sensory neurons detect information from the environment or internal tissues, motor neurons control effectors such as skeletal muscle, and interneurons connect and process signals within the Central Nervous System. The term neuron therefore describes a broad cellular category rather than one uniform cell type. Neuronal specialisation reflects the particular information each cell receives, processes and transmits.
Neuronal membranes maintain electrical gradients because ion concentrations differ between the intracellular and extracellular environments. Ion pumps and transporters establish these gradients, while ion channels permit selective movement of charged particles. The resting membrane potential results from the combined effects of ion concentration gradients and selective membrane permeability. When sufficiently strong inputs alter membrane voltage to threshold, voltage gated ion channels can generate an action potential.
The action potential is a rapid transient change in membrane voltage that propagates along the axon without progressively diminishing under normal conditions. Myelination can greatly increase conduction velocity by restricting current flow and concentrating voltage gated channels at nodes between myelinated segments. This arrangement permits saltatory conduction, in which the electrical signal effectively moves between nodes rather than travelling continuously along every part of the membrane.
Neurons communicate at synapses, which are specialised junctions where one cell influences another. At chemical synapses, an arriving action potential opens voltage dependent calcium channels, calcium enters the presynaptic terminal and triggers vesicle fusion and neurotransmitter release. Neurotransmitters cross the synaptic cleft and bind receptors on the postsynaptic membrane. Ionotropic receptors directly influence ion channels and generally act rapidly, while metabotropic receptors activate intracellular signalling pathways and often produce slower longer lasting effects. Excitatory postsynaptic potentials increase the likelihood of postsynaptic firing under appropriate conditions, whereas inhibitory potentials reduce it. A single neuron can receive thousands of synaptic inputs, and its response depends on the spatial and temporal integration of these signals.
Neurons are structurally diverse because different functions require different architectures. Multipolar neurons have many dendrites and generally one axon and are common in the Central Nervous System. Bipolar neurons possess two main processes and occur in specialised sensory pathways, while pseudounipolar neurons are common among primary sensory neurons associated with spinal sensory ganglia. Axons can be extremely short or extend over very long distances, and dendritic structures can be highly elaborate.
Some neurons form local circuits, while others connect distant brain spinal or peripheral regions. The morphology of a neuron therefore reflects its connectivity and information processing role. Neuronal populations can also differ in neurotransmitter expression, receptor composition, ion channel distribution and firing properties, producing a huge range of functional specialisation.
Neuronal communication is central to sensory processing. Receptors can transform mechanical chemical thermal or electromagnetic stimuli into changes in membrane potential, a process called sensory transduction. Sensory neurons encode information through patterns of electrical activity, including firing frequency timing and population activity. The brain and spinal cord interpret these signals according to pathway organisation and context. Motor neurons transform central commands into signals that activate muscles, while autonomic neurons influence smooth muscle cardiac muscle and glands. Interneurons provide much of the processing capacity of neural circuits by combining inputs and controlling the activity of other neurons. This organisation allows nervous systems to perform reflexes perception movement learning and regulation through networks of interconnected cells rather than isolated neurons.
Neurons require substantial metabolic support. Maintaining ion gradients and restoring membrane conditions after repeated action potentials requires ATP, while synaptic transmission and neurotransmitter recycling also consume energy. Mitochondria are therefore abundant in regions with high metabolic demand, particularly axon terminals and other active compartments. Neurons depend on oxygen and metabolic substrates delivered by the circulation and supported by interactions with glial cells.
Unlike many other cell types, mature neurons in many regions of the adult human Central Nervous System have limited capacity for cell division. This contributes to the difficulty of replacing neurons after severe injury. Some peripheral neurons can survive injury and regenerate axons under favourable conditions, particularly when supportive structures remain intact.
Neurons display plasticity throughout development and adulthood. Synaptic connections can strengthen weaken appear or disappear, and dendritic structures can change in response to experience and neural activity. Long term potentiation and long term depression are examples of mechanisms associated with persistent changes in synaptic strength. Plasticity contributes to learning memory adaptation and recovery from some injuries. However, neural plasticity can also produce maladaptive changes, including persistent pain pathways or abnormal circuit activity. Neurons interact continuously with glial cells, extracellular molecules and other cellular structures that influence their survival connectivity and activity. Neuronal function is therefore not determined solely by intrinsic electrical properties but emerges from the broader cellular environment in which each neuron operates.
Neuronal dysfunction contributes to many neurological disorders. Damage to axons can interrupt communication even when neuronal cell bodies survive, while loss of neurons can permanently disrupt circuits when replacement is limited. Abnormal excitability can contribute to seizures, while degeneration of particular neuronal populations contributes to several neurodegenerative conditions. Disorders affecting myelin can slow or block action potential conduction, and abnormalities of neurotransmitter systems can alter movement mood cognition or autonomic function. Neurons can also be damaged by inadequate blood supply, toxins metabolic disturbances inflammation trauma or excessive excitatory signalling. Research into neuronal disease examines ion channels synaptic proteins neurotransmitters mitochondrial function axonal transport inflammation and mechanisms of cell death.
